Sealed, thermally insulated tank with compression-resistant non-conducting elements

ABSTRACT

Sealed, thermally insulated tank consisting of tank walls fixed to the load-bearing structure ( 1 ) of a floating structure, said tank walls having, in succession, in the direction of the thickness from the inside to the outside of said tank, a primary sealing barrier ( 8 ), a primary insulating barrier ( 6 ), a secondary sealing barrier ( 5 ) and a secondary insulating barrier ( 2 ), at least one of said insulating barriers consisting essentially of juxtaposed non-conducting elements, each non-conducting element including a thermal insulation liner ( 63 ) and load-bearing elements that rise through the thickness of said thermal insulation liner in order to take up the compression forces, characterized in that the load-bearing elements of a non-conducting element include pillars ( 65 ) of small transverse section as compared to the dimensions of the non-conducting element in a plane parallel to said tank wall.  
     Sealed, thermally insulated tank has tank walls fixed to the load-bearing structure ( 1 ) of a floating structure, the tank walls having, in succession, in the direction of the thickness from the inside to the outside of the tank, a primary sealing barrier ( 8 ), a primary insulating barrier ( 6 ), a secondary sealing barrier ( 5 ) and a secondary insulating barrier ( 2 ), at least one of the insulating barriers includes juxtaposed non-conducting elements, each non-conducting element including a thermal insulation liner ( 63 ) and load-bearing elements that rise through the thickness of the thermal insulation liner in order to take up the compression forces, characterized in that the load-bearing elements of a non-conducting element include pillars ( 65 ) of small transverse section as compared to the dimensions of the non-conducting element in a plane parallel to the tank wall.

The present invention relates to the production of sealed, thermallyinsulated tanks consisting of tank walls fixed to the load-bearingstructure of a floating structure suitable for the production, storage,loading, ocean carriage and/or unloading of cold liquids such asliquefied gases, particularly those with a high methane content. Thepresent invention also relates to a methane carrier provided with a tankof this type.

Ocean carriage of liquefied gas at very low temperature involves anevaporation rate per day's sailing that it would be advantageous tominimize, which means that the thermal insulation of the relevant tanksshould be improved.

A sealed, thermally insulated tank consisting of tank walls fixed to theload-bearing structure of a ship has already been proposed, said tankwalls having, in succession, in the direction of the thickness from theinside to the outside of said tank, a primary sealing barrier, a primaryinsulating barrier, a secondary sealing barrier and a secondaryinsulating barrier, at least one of said insulating barriers consistingessentially of juxtaposed non-conducting elements, each non-conductingelement including a thermal insulation liner arranged in the form of alayer parallel to said tank wall, and load-bearing elements that risethrough the thickness of said thermal insulation liner in order to takeup the compression forces.

For example, in FR-A-2 527 544 these insulating barriers consist ofclosed parallelepipedal caissons made from plywood and filled withperlite. On the inside, the caisson includes parallel load-bearingspacers interposed between a cover panel and a base panel in order towithstand the hydrostatic pressure exerted by the liquid contained inthe tank. Non-load-bearing spacers made from plastic foam are placedbetween the load-bearing spacers in order to maintain their relativepositioning. Manufacture of a caisson of this type, including theassembly of the outer walls made from plywood sections and the fittingof the spacers, requires a number of assembly operations, particularlystapling. Furthermore, the use of a powder such as perlite complicatesthe manufacture of the caissons because the powder produces dust. Thus,it is necessary to use high-quality and therefore expensive plywood sothat the caisson is well sealed against dust, i.e. knot-free plywood.Furthermore, it is necessary to tamp down the powder with a specificpressure in the caisson, and it is necessary to circulate nitrogeninside each caisson in order to evacuate all the air present, for safetyreasons. All these operations complicate manufacture and increase thecost of the caissons. Moreover, if the thickness of the insulatingcaissons is increased with an insulating barrier, the risk of the wallsof the caissons and the load-bearing spacers buckling increasesconsiderably. If it is desired to increase the anti-buckling strength ofthe caissons and of their internal load-bearing spacers, the crosssection of said spacers has to be increased, which increases the thermalbridges established between the liquefied gas and the load-bearingstructure of the ship by the same amount. Furthermore, if the thicknessof the caissons is increased it is observed that, inside the caissons,gas convection currents arise that are highly detrimental to goodthermal insulation.

FR-A-2 798 902 describes other thermally insulated caissons designed foruse in such a tank. Their method of manufacture consists in alternatelystacking a plurality of low-density foam layers and a plurality ofplywood panels, placing adhesive between each foam layer and each paneluntil the height of said stack corresponds to the length of saidcaissons, in cutting the above-mentioned stack into sections in thedirection of the height, at regular intervals corresponding to thethickness of a caisson, and in adhesively bonding a base panel and a toppanel made from plywood on either side of each stack section thus cut,said panels extending perpendicularly to said cut panels, which serve asspacers. Although the result of this is a good compromise in terms ofanti-buckling strength and thermal insulation, it has to be admittedthat this manufacturing process also requires numerous assembly stages.

An object of the invention is to propose a tank of this type while alsoimproving at least one of the following characteristics withoutdetriment to others of these characteristics: the tank's cost price, theability of the walls to withstand pressure and the thermal insulation ofthe walls. A further object of the invention is to propose a tank ofthis type in which the non-conducting elements are easily adaptable interms of their dimensions, without compromising the ability of the wallsto withstand pressure and the thermal insulation of the walls.

To that end, a subject of the invention is a sealed, thermally insulatedtank including at least one tank wall fixed to the hull of a floatingstructure, said tank wall having, in succession, in the direction of thethickness from the inside to the outside of said tank, a primary sealingbarrier, a primary insulating barrier, a secondary sealing barrier and asecondary insulating barrier, at least one of said insulating barriersconsisting essentially of juxtaposed non-conducting elements, eachnon-conducting element including a thermal insulation liner arranged inthe form of a layer parallel to said tank wall, and load-bearingelements that rise through the thickness of said thermal insulationliner in order to take up the compression forces, characterized in thatsaid load-bearing elements of a non-conducting element include pillarsof small transverse section as compared with the dimensions of thenon-conducting element in a plane parallel to said tank wall.

Small-cross section pillars of this type have the advantage that theycan be distributed in the non-conducting element as a function of localrequirements. By adapting the number and the distribution of theload-bearing pillars, the non-conducting element's compression strengthcan, in particular, be made more uniform than with prior-art spacers. Itis also possible to prevent localized depression or pinching of a coverpanel. Advantageously, said pillars are regularly distributed over theentire surface of the non-conducting element seen in a plane parallel tothe tank wall. A further advantage of the non-conducting element withsmall-cross section pillars is that it allows the manufacture of anon-conducting element of any desired dimensions without loss ofcompression strength, at least insofar as these dimensions remaingreater than or equal to the spacing between the pillars. Anon-conducting element of small surface area may, in particular, beobtained by cutting an element of larger surface area.

According to a particular embodiment, said pillars are identicallyspaced apart in the length direction and in the width direction of thenon-conducting element.

Pillars of this type may have a hollow or solid cross section, for whicha number of shapes are possible. Preferably, said pillars have a closedhollow transverse section. Such hollow pillars with a closed transversesection, in particular tubes with a circular cross section, make itpossible to obtain very good anti-buckling resistance while at the sametime minimizing the effective thermal conduction cross section.

Advantageously, said pillars are produced from plastic or a composite.

Preferably, said insulation liner of the non-conducting element includesa block of synthetic foam.

According to one embodiment, said pillars are inserted in holes machinedin said block of synthetic foam.

According to a further embodiment, said block of synthetic foam isobtained by pouring between said pillars so as to embed at least oneheight portion of said pillars, for example half or all their height, insaid block of synthetic foam.

Advantageously, said non-conducting element includes a planarpositioning element arranged parallel to said tank wall in the thicknessof the insulation liner and having openings traversed by said pillars inorder to define their mutual positioning.

Preferably, said non-conducting element includes at least one panelextending parallel to said tank wall on a side of said non-conductingelement. In other words, in such a case, the non-conducting elementcomprises a base panel or a cover panel. By convention, “cover” is thename given to a panel on that side of the non-conducting element thatfaces toward the inside of the tank and “base” is the name given to apanel on the side of the non-conducting element that faces toward theload-bearing structure. The non-conducting element may also include botha base panel and a cover panel. Any fixing means may be used for fixinga panel of this type to the non-conducting element.

The non-conducting elements may be open or closed. Advantageously, thepresence of a cover panel provides uniform support for the adjacentsealing barrier. However, a panel of this type is not mandatory becausesufficient support of this type may also be obtained from the pillarsalone. Advantageously, the presence of a base panel provides welldistributed transmission of compression forces from the primaryinsulating barrier toward the secondary insulating barrier or from thesecondary insulating barrier toward the hull. However, a panel of thistype is not mandatory because this transmission may also be sufficientlyguaranteed by the pillars alone. Panels of this type may be formed inseveral ways. One possibility is to form a load-bearing structureincorporating, as a single piece, a panel with the pillars. A furtherpossibility is to fix a separate panel on a side of the non-conductingelement.

Advantageously, the inner face of a said panel has recesses arranged insuch a manner as to interact by flush-fitting with said pillars. Thisresults in a particularly robust link. In such a case, the panel mayhave a thermal expansion coefficient that is different from that of saidpillars so as to give rise to gripping between said panel and saidpillars flush-fitted in the latter when the tank is cooled.

According to a particular embodiment, said non-conducting element hasthe form of a closed box with a base panel, a cover panel and peripheralwalls extending between said panels along the edges of the latter. Adesign of this type allows the fitting of an insulation liner in theform of granular material. However, depending on the construction of theinsulation liner, it is possible, also, to use non-conducting elementsthat do not have peripheral walls.

According to a further particular embodiment, said load-bearing elementsof a non-conducting element are produced in the form of at least oneload-bearing structure formed as a single piece including, on eachoccasion, linking means that rigidly link said load-bearing elementstogether and at least one height portion of said pillars.

A load-bearing structure of this type formed as a single piece combinesvery advantageous mechanical properties both in terms of stiffness andin terms of anti-buckling resistance in the direction of the thicknessof the hollow elements, of ease of forming, of thermal insulation and ofcost price. Indeed, for a given geometry of the pillars, theiranti-buckling resistance is increased by the rigid integral links ascompared to separate pillars. Furthermore, manufacture of the linksbetween the pillars and pillars, i.e. at least one portion of theirheight, in the form of a single piece makes it possible to dispense withcertain assembly operations, makes it possible to obtain a relativelyrigid load-bearing structure without excessively increasing the crosssection of the pillars and/or their thickness, and thus the thermalbridges, and simplifies fitting of the thermal insulation liner in thenon-conducting element.

According to a further embodiment of the linking means, said linkingmeans include arms extending between said pillars. Advantageously, saidarms extend parallel to said tank wall along at least one side of saidinsulation liner. Positioned in this way, the arms offer a supplementarysurface, in addition to that of the pillars, for the fixing of apossible base panel and/or cover panel formed independently of theload-bearing structure.

According to a preferred embodiment of the linking means, said linkingmeans of a load-bearing structure include a panel extending parallel tosaid tank wall on a side of said non-conducting element, said pillarsprojecting from an inner face of said panel.

According to one embodiment of the non-conducting element, it has twoload-bearing structures arranged in such a manner that their respectivepanels have said inner faces turned toward one another, the pillarsprojecting from said inner faces being assembled in pairs in the regionof their ends located opposite said panels in order to form, on eachoccasion, a pillar of said non-conducting element. In other words, insuch a case, the pillars of each of the two load-bearing structures areplaced end to end in order to form, on each occasion, a pillar havingtwo parts extending, respectively, through a portion of the thickness ofthe non-conducting element. In particular, it is possible to use twocompletely symmetrical load-bearing structures.

Advantageously, an insulation piece having a thermal conductivity thatis lower than that of said pillars is interposed, on each occasion,between the two assembled pillars. This makes it possible to improve thethermal insulation obtained by means of the non-conducting element.

The two load-bearing structures may be assembled by any means.Preferably, the pillars of the two load-bearing structures are assembledin pairs, on each occasion, by means of a linking piece having a thermalexpansion coefficient that is different from that of said pillars so asto give rise to gripping between said linking piece and said pillarswhen the tank is cooled. As a variant embodiment, or in combination, thelinking piece may also be flush fitted, adhesively bonded, snap-fitted,etc.

Preferably, the load-bearing structure or structures of a non-conductingelement is (or are) manufactured using a process of molding, extrusion,pultrusion, thermoforming, blow-molding, injection-molding or rotationalmolding. The load-bearing structures may be manufactured from anymaterial suitable for the above-mentioned processes, particularlyplastics such as PC, PBT, PA, PVC, PE, PS, PU and other resins.Advantageously, the load-bearing structures are produced from acomposite material. The use of this type of materials brings togetherthe conditions necessary for obtaining load-bearing elements with athinner wall thickness than with plywood, while at the same timeoffering better or equivalent thermal conductivity and a lower expansioncoefficient. For example, said load-bearing structures may be producedfrom a polymer-resin-based composite material, for example polyesterresin or another resin. Within the meaning of the invention,polymer-resin-based composite materials include polymers or mixtures ofpolymers with all kinds of fillers, additives, reinforcements or fibers,for example glass fibers or other fibers, providing sufficient rupturestrength and rigidity and other properties. Additives may also beemployed to reduce the material's density and/or improve its thermalproperties, particularly reducing its thermal conductivity and/or itsexpansion coefficient. Use may also be made of a composite that includesa high proportional of sawdust with a synthetic binder. In certainembodiments, the load-bearing structure may also be made from laminatedwood or plywood molded by hot compression.

According to a particular embodiment, said at least one insulatingbarrier consisting of said non-conducting elements is covered, on eachoccasion, by one of said sealed barriers that is formed from thin metalplate strakes with a low expansion coefficient, the edges of which areraised toward the outside of said non-conducting elements, saidnon-conducting elements having cover panels carrying parallel groovesspaced by the width of a plate strake in which weld supports areslideably retained, each weld support having a continuous wingprojecting from the outer face of the cover panel and on whose two facesthe raised edges of two adjacent plate strakes are welded in a leaktightmanner. The sliding weld supports form gliding joints allowing differentbarriers to move relative to one another through the effect ofdifferences in thermal contraction and movements of the liquid containedin the tank.

Advantageously, secondary retention members integral with theload-bearing structure of the ship fix the non-conducting elementsforming the secondary insulating barrier against said load-bearingstructure, and primary retention members linked to said weld supports ofthe secondary sealing barrier retain said primary insulating barrieragainst the secondary sealing barrier, said weld supports retaining saidsecondary sealing barrier against the cover panels of the non-conductingelements of the secondary insulating barrier. Thus, the primaryinsulating barrier is anchored on the secondary insulating barrier, withno effect on the continuity of the secondary sealing barrier interposedbetween them.

According to a preferred embodiment, said thermal insulation linerincludes reinforced or unreinforced, rigid or flexible foam of lowdensity, i.e. under 60 kg/m³, for example around 40 to 50 kg/m³, whichhas very good thermal properties. It is also possible to use a materialof nanoscale porosity of the aerogel type. A material of the aerogeltype is a low-density solid material with an extremely fine and highlyporous structure, possibly with a porosity up to 99%. The pore size ofthese materials is typically in the range between 10 and 20 nanometers.The nanoscale structure of these materials greatly limits the mean freepath of the gas molecules, and therefore also convective heat and masstransfer. Aerogels are thus very good thermal insulators, with a thermalconductivity, for example, below 20×10⁻³ W.m⁻¹.K⁻¹, preferably less than16×10⁻³ W.m⁻¹.K⁻¹. They typically have a thermal conductivity 2 to 4times as low as that of other, conventional insulators, such as foams.Aerogels may be in different forms, for example in the form of powder,beads, nonwoven fibers, fabric, etc. The very good insulating propertiesof these materials make it possible to reduce the thickness of theinsulating barriers in which they are used, which increases the usefulvolume of the tank.

The invention also provides a floating structure, in particular amethane carrier, characterized in that it comprises a sealed, thermallyinsulated tank according to the subject of the above invention. A tankof this type may, in particular, be employed in an FPSO (floating,production, storage and offloading) facility, used to store theliquefied gas with a view to exporting it from the production site, oran FSRU (floating storage and regasification unit) used to unload amethane carrier with a view to supplying a gas transportation system.

The invention will be better understood and further objects, details,characteristics and advantages thereof will become more clearly apparentin the course of the following description of a particular embodiment ofthe invention that is given solely by way of non-limiting illustrativeexample with reference to the appended drawings, in which:

FIG. 1 is a stripped-back perspective view of a tank wall according to ageneral embodiment that is useful for understanding the invention;

FIGS. 2 and 3 show a primary retention member of the tank wall of FIG. 1seen in two perpendicular directions;

FIG. 4 is a transverse sectional view of a tank wall according to oneembodiment of the invention;

FIG. 5 is an expanded perspective view of a non-conducting element ofthe tank wall shown in FIG. 4;

FIG. 6 is a perspective view of a molding step for obtaining anon-conducting element according to the first embodiment of theinvention;

FIG. 7 shows, in perspective, a load-bearing structure molded as asingle piece;

FIG. 8 is a partial sectional view showing a variant embodiment of theload-bearing structure of FIG. 7;

FIG. 9 is an expanded perspective view of two types of non-conductingelement produced with the aid of the load-bearing structure of FIG. 7;

FIG. 10 is a partial, sectional view showing the assembly of anon-conducting element of FIG. 9;

FIGS. 11 and 12 are views similar to FIG. 7, showing other variantembodiments of the load-bearing structure;

FIG. 13 is a partial, sectional view of a non-conducting elementaccording to a further embodiment of the invention;

FIG. 14 is a plan view of the load-bearing structure of thenon-conducting element of FIG. 13;

FIGS. 15 to 18 show further embodiments of load-bearing elements in theform of pillars, seen in transverse section;

FIG. 19 is a view similar to FIG. 6, showing an alternate moldingmethod;

FIG. 20 is an expanded perspective view of a non-conducting elementaccording to a further embodiment of the invention;

FIG. 21 shows, in perspective, a load-bearing structure thermoformedfrom a single piece; and

FIGS. 22 and 23 show in plan view and in sectional view on line XXIII anon-conducting element according to a further embodiment.

A description will be given below of several embodiments of a sealed,thermally insulated tank incorporated in and anchored to the double hullof a structure of the FPSO or FSRU type or of a methane-type carrier.The general structure of such a tank is well known per se and has apolyhedral form. Therefore, a description will be given only of a wallzone of the tank, it being understood that all the walls of the tankhave a similar structure.

A description is now given of a general embodiment that is useful forunderstanding the invention, with reference to FIGS. 1 to 3. FIG. 1shows a zone of the double hull of the ship, denoted by 1. The tank wallis composed, in succession, in its thickness, of a secondary insulatingbarrier 2 formed from caissons 3 juxtaposed on the double hull 1 andanchored to the latter by means of secondary retention members 4, then asecondary sealing barrier 5 carried by the caissons 3, then a primaryinsulating barrier 6 formed from juxtaposed caissons 7 anchored to thesecondary sealing barrier 5 by primary retention members 48, and finallya primary sealing barrier 8 carried by the caissons 7.

The caissons 3 and 7 are parallelepipedal non-conducting elements with amutually identical or different structure and mutually identical ordifferent dimensions.

Secondary retention members 4 are fixed on pins 31 welded to the doublehull 1 in a regular rectangular grid arrangement so that these retentionmembers 4 can, on each occasion, hold four caissons 3, whose cornersmeet. Also provided are two secondary retention members 4 in the centralzone of each caisson 3. However, depending on the size of the caisson,more or fewer than six anchoring points per caisson 3 may be necessary.

The secondary sealing barrier 5 is produced in accordance with the knowntechnique in the form of a membrane consisting of Invar plate strakes 40with raised edges. As may be seen better in FIG. 3, the cover panels 11of the caissons 3 have longitudinal grooves, with an inverted-T-shapedcross section, denoted by 41. A weld support 42 in the form of a stripof Invar folded in the form of an L, is inserted slideably in eachgroove 41. Each plate strake 40 extends between two weld supports 42 andhas two raised edges 43 welded, on each occasion, continuously by a weldbead 44 to the corresponding weld support 42, as may be seen in FIGS. 2and 3. The primary sealing barrier 8 is produced in the same manner.Similarly, the caissons 7 of the primary insulating barrier areanchored, on each occasion, to the four corners and at two points in thecentral zone of the caisson 7. To that end, use is made, on eachoccasion, of a primary retention member 48 shown in detail in FIGS. 2and 3. The primary retention member 48 has a lower sleeve 49 integralwith a lug 50 welded at several, for example three, points 51 of a weldsupport 42 above the raised edges 43 of the plate strakes 40. A rod 52made from Permali, a composite material based on resin-impregnated beechwood, has a lower end fixed in the lower sleeve 49 and an upper endfixed in a sleeve 54 integral with a support washer 53 that bears on thecover panels 11 of the caissons 7, being accommodated in countersinks 28at the corners of the caissons 7 and at the central shafts 30. Thesleeve 54 is threaded and is screwed onto a corresponding threaded endof the rod 52. When the washer 53 has been thus positioned, immobilizingscrews 56 are engaged through holes 55 provided in the washer 53 andscrewed into the panel 11 in order thus to prevent any subsequentrotation of the washer 53. In each insulating barrier, the caissons 3and 7 are juxtaposed with a small intermediate space of the order of 5mm.

Advantageously, a layer of nanoporous materials of the aerogel type,which are very good thermal insulators, is included as insulation linerin the caissons 3 and/or 7. Aerogels also have the advantage of beinghydrophobic, so absorption of the moisture from the boat into theinsulating barriers is thus prevented. An insulation layer may beproduced with aerogels, possibly pocketed, in textile form or in theform of beads.

Generally speaking, aerogels may be made from a number of materials,including silica, alumina, hafnium carbide and also varieties ofpolymers. Furthermore, in accordance with the manufacturing process,aerogels may be produced in powder, bead, monolithic sheet andreinforced flexible fabric form. Aerogels are generally manufactured byextracting or displacing the liquid of a gel of micronic structure. Thegel is typically manufactured by means of chemical conversion andreaction of one or more dilute precursors. This results in a gelstructure in which a solvent is present. Use is generally made ofhypercritical fluids such as CO₂ or alcohol, to displace the gelsolvent. Aerogels' properties may be modified by using a variety ofdoping and reinforcement agents.

The use of aerogels as insulation liners significantly reduces thethickness of the primary and secondary insulating barriers. It is, forexample, possible to conceive of barriers 2 and 6 having a thickness of200 mm and 100 mm, respectively, by using an aerogel bed in textile formin the caissons 3 and 7. The tank wall then has a total thickness of 310mm. As a variant embodiment, it is possible to conceive of a tank wallhaving a total thickness of 400 mm by using, on each occasion, a layerof aerogel particles, particularly aerogel beads, in the caissons 3 and7.

With reference to FIGS. 4 and 5, a description will now be given of afirst embodiment of a sealed, thermally insulated tank according to theinvention. In the first embodiment, the primary and secondary insulatingbarriers are formed from non-conducting elements in the form ofparallelepipedal caissons 60 whose structure is shown in FIG. 5 and thatare arranged and anchored in a similar manner to the caissons 3 and 7 ofFIG. 1, so a further description is unnecessary in this regard.

The caisson 60 includes a block of low-density synthetic foam 63, forexample low-density polyurethane foam, optionally reinforced withfibers, sandwiched between a base panel 61 and a cover panel 62 that arefixed to its larger faces, for example by means of adhesive bonding.

Between the panels 61 and 62, load-bearing pillars 65 in the form ofhollow tubes with a circular cross section extend in holes 64 providedin the thickness of the block 63. In the example shown, the pillars 65are distributed in the form of a square-mesh grid, but other forms ofdistribution are possible. In the case of a non-conducting element witha 1.5-m-sided square cross section, provision is made, for example, forsixty-four pillars 65. However, the density of the pillars may bemodified, particularly as a function of the forces to be taken up and ofthe cross section of the pillars. The inside of the pillars 65 is filledwith insulation, which is, for example, the same foam as that formingthe block 63 between the pillars 65, or another material, for example amaterial of higher density, in order to take up more compression forces.

In the embodiment of FIG. 5, the caisson 60 may be manufactured by meansof the following steps: cutting a block of foam 63 from a bed ofcontinuously-poured foam, machining holes 64 through the block 63,inserting pillars 65 in the holes 64, inserting plugs of insulation 66in the pillars 65, and adhesive bonding of the panels 61 and 62.

An alternate manufacturing method corresponds to FIG. 6, in which theblock of foam is omitted. In such a case, pillars 65 are placed in thecavity 68 of a mold 67 and then foam is poured between the pillars 65 soas to obtain a block of foam in which the pillars 65 are embedded. Thepillars 65 may also be filled during the same pouring step if theirdiameter is fairly large, for example greater than 100 mm. In order toguarantee the positioning and holding of the pillars 65 in the cavity ofthe mold, a planar positioning element is used, in this case in the formof a grid or of a glass mat 69, through which the pillars 65 are tightlyfitted. The grid or glass mat 69 is also embedded in the thickness ofthe block of foam after molding, which makes it possible to reduce theexpansion coefficient of the foam in this zone and thus to reduce theshear stresses between the panels 61 and 62 and the foam. Lastly, thepanels 61 and 62 are adhesively bonded. Alternately, or in combinationwith this adhesive bonding, it is possible to fit the panels and theends of pillars 65 together, which ends should, in such a case, extendbeyond the block 63.

It would also be possible to commence by fixing the pillars 65 on thepanel 61 and placing this assembly in the mold 67 in order to pour thefoam directly over the panel 61, with or without the grid 69.

FIG. 19 illustrates, using the same reference numerals as in FIG. 6, afurther variant embodiment of the process in which the block of foam 63is molded between the panels 61 and 62, which panels are placed with thepillars 65 (and, as appropriate, the grid or glass mat 69) in the mold67, which is closed by a cover 59. This results in a caisson 60 that isfinished in a single operation.

The pillars 65 may be manufactured in a number of materials. Plasticssuch as PVC, PC, PA, ABS, PU, PE and the like are particularly suited tothe molding of pillars of any form and have an advantageous cost price.Other possible materials are composites, wood, plywood or syntheticfoams. The panels 61 and 62 may be produced from plywood, plastic resinor a composite material. For example, their thicknesses are 6.5 mm forthe base and 12 mm for the cover.

It will be noted that the caisson 60 may be manufactured, or, above all,easily cut out, in any form whatsoever in order to achieve preciseconnections when the tank is constructed or to take up tolerances.Indeed, it is easy to cut the panels 61 and 62 and the block 63 betweenthe pillars 65 without compromising the cohesion and compressionstrength of each caisson part thus separated. As appropriate, it is alsopossible to cut hollow pillars 65 vertically.

The tank wall produced with the aid of the caissons 60 is shown insection in FIG. 4. In this example, thicker caissons are used for thesecondary insulating barrier 2 than for the primary insulating barrier6. The detail of the primary 4 and secondary 48 anchoring members and ofthe sealing barriers 5 and 8 is not shown. Reference may be made toFIGS. 1 to 3 in this regard.

As the geometry of the double hull 1 is irregular, provision is made forshims around the threaded pins 31. The thickness of each shim iscalculated by computer on the basis of a topographical survey of theinner surface of the double hull 1. Thus, the base panels 61 of thesecondary barrier 2 are positioned along a theoretical regular surface.Between the base panels 61 and the double hull 1, provision isconventionally made for beads of mastic 70 that are adhesively bonded tothe base panels 61 and are crushed against the double hull when thecaissons 60 are fitted, so as to provide their support. To avoid thismastic adhering to the double hull, a sheet of Kraft paper (not shown)is provided between them. Preferably, the beads 70 are placed in linewith the pillars 65 in order to prevent flexing of the panel 61 onaccount of the compression force, which is transmitted predominantly inthe region of the pillars 65. Furthermore, it would be possible todispense with base panels and to rest the pillars 65 directly on thebeads 70.

According to a variant embodiment (not shown), provision is made forperipheral walls extending to the periphery of the caisson 60 betweenthe panels 61 and 62 so as to form a closed box capable of containinggranular insulation. These walls may be fixed to the panels by means ofadhesive bonding, stapling, flush-fitting and other fixing means. Thecaisson 60 may also be assembled in monobloc fashion, for example bymeans of blow-molding or rotational molding.

According to a further variant embodiment, the panels 61 and/or 62 arereplaced by panel portions that cover only zones of the block 63 at theend of the pillars 65, not the entire surface of the block 63. The weldsupports 42 will then be housed in the cover-panel portions.

Provision may be made for oblique pillars 65, i.e. pillars whose axis isnot perpendicular to the base 61 and cover 62 panels. An inclination ofthis type makes it possible to take up not only shear forces but alsooverturning forces applied to the caisson 60. With reference to FIGS. 7to 12, a description is given of further embodiments of non-conductingcaissons or elements that can be used to form the insulating barriers ofthe tank wall, the general structure of which was described for FIGS. 1to 3. The production of the sealing barriers and the attachment of thevarious barriers is similar to the preceding embodiments, there will beno point in describing them again here.

FIG. 9 shows, in expanded perspective view, a caisson 570 and a caisson670 that are, respectively, manufactured with the aid of moldedload-bearing structures 500, a description of which will now be givenwith reference to FIG. 7.

The load-bearing structure 500 is an injection-molded piece made fromany appropriate material. It has a flat plate 571 with chamferedcorners, for example in the form of a 1.5-m-sided square or of arectangular, from one face of which sixteen hollow circular cylindricalpillars 575 project, arranged in the form of a regular square grid, plustwo tubes 581 of smaller cross section in the region of a central zoneof the plate, and also four triangular cylindrical pillars 580 in theregion of the four corners of the plate. The plate 571 is continuous inthe region of the base of the pillars 575 and 580, but pierced in theregion of the base of the tubes 581 in order to allow the passage of acoupler rod. Furthermore, in the case of a caisson of the primarybarrier 6, the plate 571 is slit in order to allow through the weldsupports 42 and the raised edges 43 of plate strakes of the secondarysealing barrier. The pillars 580 serve to receive the bearing forces ofthe coupling members used at each corner of the non-conducting elements.The cross section of the pillars 575 is, for example, 300 mm for a 1.5 msquare plate. As for the insulating liner, the load-bearing structure500 may be covered with a layer of low-density foam, which is pouredbetween and into the pillars 575.

The cross section of the pillars may be reasonably large, the importantthing being to always make provision for several pillars per caisson.Thus, the dimensions of the pillars in terms of cross section may be ⅓or even ½ of the corresponding dimensions of the caisson.

In order to form the caisson 570, an independent panel 572 with the samedimensions as the plate 571 is fixed on the end of the pillars 575opposite this plate. This panel may be fixed by any means (adhesivebonding, stapling, flush fitting, etc.). In FIG. 9, provision has beenmade for circular grooves 573 on the inner face of the panel 572, forreceiving the end of each pillar 575 tightly.

The materials of the structure 500 and of the panel 572 may be chosen soas to produce heat-shrinking of the pillars 575 in the panel. Forexample, with a piece 500 made from PVC and a panel 572 made fromplywood, which exhibits less heat shrinkage, the end of the pillars 575is made to grip the circular core delimited by the groove 573 when thetank is cooled. Conversely, gripping of the pillars 575 could also beobtained with a panel 572 that contracts more than the piece 500.

The panel 572 has holes 574 opposite the tubes 581 of the moldedstructure 500.

In the caisson 670, two identical molded structures 500 are arrangedsymmetrically and assembled together by causing their respective pillars575 to bear against one another. This assembly may be produced by anymeans (adhesive bonding, welding, flush fitting, etc.). In FIG. 9, it isachieved with the aid of a linking ring 680 interposed, on eachoccasion, between two aligned pillars 575 and flush fitted over them.This assembly can be seen better in FIG. 10, where it will be observedthat the linking ring 680 has an outer annulus 682 and an inner annulus681 that are connected by means of a radial tongue 683. The pillars 575flush fit between the two annuli 681 and 682 and abut on either side ofthe tongue 683. The material of the ring 680 may be chosen to have lowerconductivity than that of the pillars 575, in order to fulfill a thermalinsulation function. They may also, alternately or in combination, bechosen to have an expansion coefficient that is different from that ofthe pillars 575 in order to fulfill a thermal assembly function. In avariant embodiment, two molded structures having pillars withcomplementary cross sections may be fixed together by means of directnesting of the pillars together.

The foam-filled piece 500 may also be used alone without a supplementarypanel by rotating the plate 571 toward the inside of the tank in orderto support the adjacent sealing barrier. The non-conducting element thusformed rests via the pillars 575 on the secondary sealing barrier or onthe strips of resin fixed to the hull.

FIGS. 11 and 12 show molded load-bearing structures 600 and 700 thatmake it possible to produce non-conducting elements in a manner similarto the structure 500 described previously.

In FIG. 11, identical reference numerals to those in FIG. 7 denoteidentical elements. The structure 600 includes planar peripheral walls601 extending continuously along the four edges of the plate 571,forming a box capable of containing insulation in the form of powder,beads or the like. For example, a structure 600 containing aerogel beadsmay be combined with a structure 600 containing low-density foam to forma caisson 670 as shown in FIG. 9.

In FIG. 12, the planar plate 771 carries thirty-six hollow tubularpillars 775 of smaller cross section (for example 100 mm) than theabove-mentioned pillars 575, four hollow tubular pillars 780 with aneven smaller cross section (for example 50 to 60 mm) in the region ofits corners, and two tubular pillars 781, similar to the pillars 780, inthe region of a central zone of the plate 771 in order to allow thecoupling members serving to attach the insulating barrier to passthrough.

The structures 500, 600 and 700 may be injection-molded. A similarstructure may also be obtained by thermoforming from a plastic plate.This possibility is illustrated in FIG. 8. In such a case, the initiallyplanar plate 571 is heated and deformed to match the impression of afemale mold 560. This results in load-bearing pillars 575 whoseplate-side end is open and whose opposite end is closed by a wall 583.In such a case, the space 582 located inside the pillars 575 is filledwith, for example, foam from the face of the plate 571 opposite thesepillars.

The walls 601 may also obtained by thermoforming.

FIG. 21 shows, in perspective, a thermoformed load-bearing structure1300 that includes a plate 1371 that can act as base panel or coverpanel for a caisson, and load-bearing pillars 1375 obtained in a similarway to the pillars 575 in FIG. 8. In the example shown, the pillars 1375have a frustoconical shape, which facilitates their forming. Forexample, provision may be made for a pillar diameter that varies from160 mm at the base to 120 mm at the top, over a height of approximately100 mm.

In order to serve as base panel of a caisson of the primary insulatingbarrier, the plate 1371 is provided with two longitudinal ribs 1384extending over the entire length of the plate 1371. Each rib 1384 isobtained during the thermoforming operation by pushing the material inthe same direction as the pillars 1375, so as to form a V-shaped foldthat is open on the planar face of the plate 1371, the inner space 1385of which allows the weld supports 42 and the raised edges 43 of thesecondary sealing barrier to pass through. In the case of the secondaryinsulating barrier, the ribs 1384 are unnecessary.

A description was given previously of the load-bearing structures thatinclude a plate acting as cover or base panel. A description is nowgiven of a further embodiment of a non-conducting element 870 withreference to FIG. 13, in which the molded load-bearing structure 800includes load-bearing elements 875 of small cross section connected byarms 890. This load-bearing structure is in plan view in FIG. 14. Theload-bearing elements 875 are hollow circular cylindrical pillarsarranged in a regular grid and connected by arms 890 that are arrangedin the form of a square-mesh grid. A cover panel 872 and a base panel871, for example made from plywood, plastic, composite or anothermaterial, are adhesively bonded on the two faces opposite theload-bearing structure 800. The arms 890 are located at the end of theload-bearing elements 875 adjacent to the panel 872 and have a planarupper face, which may serve for adhesive bonding of the panel 872.

FIG. 20 shows the non-conducting element 870 in expanded perspectiveview, in a version that its slightly modified in terms of thearrangement of the linking arms 890.

Other arms may be provided in the region of the lower end of the pillars875. The arms may also be placed in another region of the load-bearingpillars (for example half way up).

The inner space of the caisson 870, i.e. the inner space 880 of thepillars 875, and the space 876 between the pillars is filled with one ormore types of insulation. When low-density foam is used, the caisson maybe manufactured by placing a structure 800 of rectangular form in planview in a mold, pouring the foam into the mold so as to embed thestructure 800 in a parallelepipedal block of foam, then fixing thepanels 872 and 871 to this block. The base panel 871 is not alwaysnecessary. One of the panels may also be molded as a single piece withthe structure 800.

Although a description has been given of hollow load-bearing pillars ofcircular cross section in the caisson 60 and the load-bearing structures500, 600, 700 and 800, the load-bearing pillars may have any other formin terms of cross section and any type of regular or irregular spatialdistribution. For example FIG. 15 shows a load-bearing pillar 975consisting of a plurality of concentric cylindrical walls 976. In thepillar 1075 of FIG. 16, the cylindrical walls 1076 have a square crosssection.

FIG. 17 shows pillars 1175 distributed in lines in the form of a regularfigure and with a hollow, square cross section with chamfered corners.In FIG. 18, pillars 1275, for example solid circular cylinders, aredistributed in a staggered arrangement. Other cross sections are alsoachievable, i.e. rectangular, polygonal, I-shaped, solid or hollow,dihedral, etc. cross sections. The load-bearing pillars may also have across section that varies over their height, for example frustoconicalpillars.

In all cases, such pillars may be molded so as to project from a plateand/or be linked by arms and/or by any linking means. When use is madeof low-density foam as thermal insulation liner layer, it isparticularly advantageous to pour this foam in a single step over theentire surface area of the linking plate, between and possibly into theload-bearing pillars. Another possibility is to machine wells in a blockof foam formed in advance and to insert the load-bearing pillars intothe wells formed for that purpose.

In the case of a granular insulation, it is necessary to use anon-conducting element with peripheral walls that are preferably formedas a single piece with the load-bearing structure, as in FIG. 11. Byvirtue of the form of the load-bearing elements of small cross section,the inner space of the box between them is not compartmentalized, andtherefore the granular material is easier to distribute over the entiresurface area of the non-conducting element. The granular material mayalso be inserted into hollow pillars. Load-bearing pillars of very smallcross section, for example smaller than 40 mm, may be left empty withoutdetriment to the thermal insulation. Hollow pillars of small crosssection may also be filled with a flexible-PE foam cone or with glasswool.

In the load-bearing structures 500, 600, 700 and 800 describedpreviously, some pillars may also be replaced by partitions creatingcompartments inside the load-bearing structure.

With reference to FIGS. 22 and 23, a description is now given of anembodiment of a non-conducting element that comprises a monobloc hollowcaisson 1470 produced by rotational molding or by injectionblow-molding. This caisson has the form of a closed hollow envelope 1477that includes eight frustoconical pillars 1475 formed so as to projectfrom the base wall 1471 of the envelope and each having a top wall 1483capable of bearing against the top wall 1472 of the envelope in order totake up the compression forces.

To fix the caisson, six frustoconical shafts 1480 are provided, arrangedat the periphery of the envelope and open through the top wall 1472.These shafts each have a base wall capable of bearing against the basewall 1471 in order take up the compression forces- and capable of beingpierced in order to receive a fixing rod, shown diagrammatically at1431, which is, for example, a pin welded to the hull or a couplingdevice fixed to an underlying sealing barrier.

The inner space 1476 of the caisson and the inner space 1482 of thepillars 1475 may be filled with any suitable insulation, for example byinjection of foam.

Similarly, the shafts 1480 may be filled with insulation, for example PEfoam or glass wool, after the caisson is fixed.

To mold the caisson 1470, use may be made, for example, of high-densityPE, polycarbonate, PBT or another plastic. The shafts 1480 may also bedispensed with if use is made of another method of attaching thecaissons, for example coupling members passing between the caissons tobe attached and bearing on the top wall 1472 in the manner of theretention members 48 of FIGS. 2 and 3. Base and/or cover panels may alsobe fixed to the walls of the envelope in order to reinforce it.

Although a description has been given of essentially parallelepipedal,right-angled non-conducting elements, other forms of cross section arepossible, notably any polygonal form capable of rendering a planarsurface discrete.

Of course, the insulation liner of a non-conducting element may includean number of layers of material.

When one of the primary and secondary insulating barriers is producedwith the aid of the non-conducting elements described above, it ispossible, but not necessary, to produce the other insulating barrier inan identical manner. Non-conducting elements of two different types maybe used in the two barriers. One of the barriers may consist ofprior-art non-conducting elements.

The caissons of the secondary insulating barrier and of the primaryinsulating barrier may be anchored to the ship's hull in a different wayfrom the example shown in the figures, for example with the aid ofretention members engaged on the base panel of the caissons.

Although the invention has been described in connection with a number ofparticular embodiments, it is obviously not limited to these in any wayand includes all technical equivalents of the means described and alsocombinations thereof if they fall within the scope of the invention.

1. Sealed, thermally insulated tank including at least one tank wallfixed to the hull (1) of a floating structure, said tank wall having, insuccession, in the direction of the thickness from the inside to theoutside of said tank, a primary sealing barrier (8) a primary insulatingbarrier (6), a secondary sealing barrier (5) and a secondary insulatingbarrier (2), at least one of said insulating barriers consistingessentially of juxtaposed non-conducting elements (3, 7), eachnon-conducting element including a thermal insulation liner (63)arranged in the form of a layer parallel to said tank wall, andload-bearing elements that rise through the thickness of said thermalinsulation liner in order to take up the compression forces,characterized in that the load-bearing elements of a non-conductingelement (60, 570, 670, 870) include pillars (65, 575, 775, 875, 975,1075, 1175, 1275) of small transverse section as compared to thedimensions of the non-conducting element in a plane parallel to saidtank wall.
 2. Sealed, thermally insulated tank according to claim 1,characterized in that said pillars are regularly distributed over theentire surface of the non-conducting element seen in a plane parallel tothe tank wall.
 3. Sealed, thermally insulated tank according to claim 1,characterized in that said pillars are identically spaced apart in thelength direction and in the width direction of the non-conductingelement.
 4. Sealed, thermally insulated tank according to claim 1,characterized in that said pillars have a closed hollow transversesection.
 5. Sealed, thermally insulated tank according to claim 4,characterized in that said pillars are tubes of circular cross section.6. Sealed, thermally insulated tank according to claim 1, characterizedin that said pillars are produced from plastic or a composite. 7.Sealed, thermally insulated tank according to claim 1, characterized inthat said insulation liner of the non-conducting element includes ablock of synthetic foam (63).
 8. Sealed, thermally insulated tankaccording to claim 7, characterized in that said block of synthetic foamis obtained by pouring between said pillars so as to embed at least oneheight portion of said pillars in said block of synthetic foam. 9.Sealed, thermally insulated tank according to claim 7, characterized inthat said pillars (65) are inserted in holes (64) machined in said blockof synthetic foam.
 10. Sealed, thermally insulated tank according toclaim 1, characterized in that said non-conducting element includes aplanar positioning element (69) arranged parallel to said tank wall inthe thickness of the insulation liner and having openings traversed bysaid pillars (65) in order to define their mutual positioning. 11.Sealed, thermally insulated tank according to claim 1, characterized inthat said non-conducting element includes at least one panel (61, 62,571, 572, 711, 872) extending parallel to said tank wall over at leastone side of said non-conducting element.
 12. Sealed, thermally insulatedtank according to claim 11, characterized in that the inner face of onesaid panel (572) has recesses (573) arranged in such a manner as tointeract by flush-fitting with said pillars (575).
 13. Sealed, thermallyinsulated tank according to claim 12, characterized in that said panel(572) has a thermal expansion coefficient that is different from that ofsaid pillars (575) so as to give rise to gripping between said panel andsaid pillars flush-fitted in it when the tank is cooled.
 14. Sealed,thermally insulated tank according to claim 11, characterized in thatsaid non-conducting element has the form of a closed box with a basepanel (571), a cover panel (572) and peripheral walls (601) extendingbetween said panels along the edges of the latter.
 15. Floatingstructure, characterized in that it comprises a sealed, thermallyinsulated tank according to claim
 1. 16. Floating structure according toclaim 15, characterized in that it consists of a methane carrier. 17.Sealed, thermally insulated tank according to claim 2, characterized inthat said pillars are identically spaced apart in the length directionand in the width direction of the non-conducting element.